This invention relates to a fiber sensor for measuring external factors such as strain, temperature or the like utilizing optical fibers, and more particularly to a fiber sensor for measuring strain or temperature by transmitting laser beams through polarization-maintaining fibers.
A strain or temperature sensor using polarization-maintaining fibers has been already developed. However, it has been found that such a temperature sensor is greatly affected by strain, while a strain sensor is affected by temperature. One example of such a sensor using polarization-maintaining fibers (for example, document No. 2F17 distributed in combination seminar of Tohoku district branches of societies concerning electricity, 1984) will be explained referring to FIG. 1.
The term "polarization-maintaining fiber" used in the specification and claims is to be understood to mean a fiber manufactured in a manner that one of two orthogonal polarization modes is unable to propagate light beams or propagation constants of the two orthogonal polarization modes are much different in order to make very small the transference of energy from one mode to the other. It may be also generally referred to as "single-polarization single-mode fiber".
In FIG. 10, reference numeral 10 denotes a polarization-maintaining fiber for use in a temperature sensor or a strain sensor. These temperature and strain sensors are substantially the same as in construction so long as FIG. 1 illustrates. The sensor comprises a laser 21 such as gas or semiconductor laser, a polarizer 22 for polarizing output beams from the laser 21 at a plane of polarization at 45.degree. (orthogonal components 1:1), a lens 23 for admitting the beams passed through the polarizer 22 into the fiber 10, an analyzer 24 for acting upon the beams from the fiber 10, a collimator 25 for collecting the beams from the analyzer 24, a photodetector 26 such as photomultiplier, photodiode and avalanche photodiode for detecting the intensity of the beams from the collimator 25, and a display unit 27 such as Brawn tube display or recorder for displaying the output of the photodetector 26.
It is considered that X and Y components of polarized light beams to be admitted into the polarizationmaintaining fibers through the polarizer 22 are equal. Owing to change in temperature (in case of the temperature sensor) or strain or tensile force in an axial direction of the polarization-maintaining fibers (in case of the strain sensor), the X and Y components of the polarized light beams pass through the polarization-maintaining fiber in different time (delay time). In other words, the time required for passing through the fibers is different for the X and Y components. Namely, the shift or difference in phase between the X and Y components of the polarized light beams at the exit end of the polarization-maintaining fiber 10 is changed so that the polarization of the resultant beams of the X and Y components changes from linear polarization to circular and elliptical polarizations. Such changes are detected by the photodetector 26 through the analyzer 24 as the change in intensity of light beams. The change in intensity is displayed in the display unit 27.
In this case, with the temperature sensor, the change in rotative angle of the polarization at the exit end of the polarization-maintaining fiber 10 is not brought about only by the change in temperature. It is also brought about by strain or tensile force in the axial direction of the polarization-maintaining fiber 10.
In the same manner, with the strain sensor, the change in rotative angle of the polarization at the exit end of the polarization-maintaining fiber 10 is not brought about only by strain or tensile force in the axial direction of the polarization-maintaining fiber 10. It is also brought about by the change in temperature. Both the effects could not be separated.
In general, on the other hand, the change in temperature causes the polarization-maintaining fiber 10 itself to expand or contract. Moreover, if there is a protective coating for the fiber 10, it would be subjected to strains of expansion or contraction of the protective coating. Accordingly, the rotation of the polarization at the exit end of the fiber 10 results in detection of changes in temperature and strain simultaneously, so that the sole temperature or strain could not be exactly measured.
Moreover, the temperature sensor or strain sensor using fibers is often used under bad circumstances. Under such conditions, the sensor would be further subjected to extra strains such as vibrations and undue change in temperature, so that the accuracy of measurement of temperature or strain further lowers.